In the field of magnetic recording, rigid-disk drive systems have enjoyed widespread popularity. A rigid-disk drive system generally includes a stack of disks mounted on a spindle which are rotated by a motor inside an enclosure. The enclosure has a controlled air supply to minimize internal contamination.
Reading and writing of binary digital information is achieved with an array of transducers, each provided with a spring suspension attached to an arm assembly. The transducers, or heads, are individually mounted on sliders which are loaded against the surface of the rotating disk medium by the spring succession. The arm-assemblies are connected to a common spindle which are positioned by an electro-magnetic actuator to provide selective access of the heads to any desired track on the disk. Movement of the electro-magnetic actuator is controlled by a signal from a control track on the disk medium. Usually one head near the center of the head stack is reserved for reading the control signal. The actuator itself is only one part of the control system which is used to control the movement of multiple head-arm assemblies across the disk surface. The control system also includes electronic circuitry, servo mechanisms, etc., which operate in concert with the actuator to position and maintain the heads over the desired data track, as well as minimizing track misregistration, during reading/writing of information.
High performance disk drive systems commonly employ a moving coil actuator (e.g., a voice coil motor) for positioning the magnetic recording heads. Coarse positioning, which involves moving the heads from one data track to another, is accomplished by controlling the actuator and transducers in a velocity feedback loop. After a coarse position move is executed, the feedback loop settles the transducers on the selected data track and continues following the data track for normal read/write operations.
An important parameter in a feedback control loop is the torque of the moving coil actuator. Ideally, the torque of a moving actuator is constant with position. That is, a linear torque renders the loop's dynamic response uniform with coil position. As the coil subtends the arc created when the actuator pivots about its spindle axis, the torque generated in the coil should be constant throughout. The linearity of the torque curve is important since it influences the access time of the magnetic recording system. Generally speaking the more linear the torque curve, the faster the access time.
Consider, by way of example, how torque affects the settling time at the end of an access. Any variation of the torque with angular position causes a variation in the open loop gain of the track following servo mechanism. The larger the magnitude of the variation in torque, the larger the resultant difference in the servo system response characteristics. Ultimately, this results in a longer, worse-case settling time.
Another problem has to do with the fact that large variations in torque can also lead to stability problems during track following.
As one might expect, in reality the torque of an actuator varies with the angular position of the coil. In other words, as the coil moves during a data stroke (e.g., movement of the transducer from an inner radial position over the magnetic disk, to an outside radial position) the force being applied to the coil varies as a function of the location of the coil on the coil arc. The data stroke comprises most of the coil arc.
The factors contributing to a non-linear torque in a disk drive system are numerous. First of all, because the disk drive enclosure geometrically bounds the actuator volume, the steel and magnetic materials which comprise the magnetic core limit the actuator magnetically to practical flux density levels. The difficulties with control of power supply and disk drive heat dissipation constrain the actuator coil current. Both of these factors present difficulties which must be overcome if a linear torque curve is to be achieved.
Furthermore, because the coil tends to be more intimate with the magnets near the center of the data stroke, as opposed to the data stroke endpoints, flux density in the center region is considerably higher than at the endpoint regions. This higher flux density generates a larger torque which, in turn, contributes to a non-linearity and longer access times. Given the numerous volumetric, magnetic and current constraints, past approaches have found it difficult to achieve a substantially linear torque curve in a rotary moving coil actuator.
The present invention covers an improvement to a moving coil actuator which renders the associated torque curve substantially linear with position. The overall flattening of the torque curve helps to improve access time for the magnetic recording system; both in the coarse positioning and in the settling portion of the access period.